1. Field of the Invention
The present invention relates generally to a vector control method and system for an induction motor and more specifically to a decoupled-vector control method and system for an induction motor, in or by which the primary current corresponding to the secondary magnetic flux and the primary current corresponding to the secondary driving current are controlled independently in such a way that these two vectors meet at right angles to each other. Here, the above-mentioned decoupled-vector implies that the mutual interference produced between the secondary magnetic flux and secondary driving current is cancelled out.
2. Description of the Prior Art
Recently, the method of driving an induction motor has been highly developed owing to a remarkable progress in power electronic device technology. Especially, the vector control method has been proposed for driving an induction motor at variable speeds under quick response characteristics equivalent to a DC machine. In this vector control method, the primary current of an induction motor is divided into a primary exciting current to generate the secondary magnetic flux and a primary driving current to generate the secondary driving current, and further the vectors of the secondary magnetic flux and the secondary driving current are so controlled independently as to meet at right angles to each other. Further, in this vector control method, the magnitude of the secondary magnetic flux is controlled at a constant level and the secondary driving current is increased or decreased independently as in a DC motor. In the above-mentioned vector control method of driving an induction motor, however, since there exists a mutual interference between the secondary magnetic flux and the secondary driving current, the magnitude of the secondary flux is not maintained constant in practice. To overcome this problem, the so-called decoupled-vector control method is adopted, in which the mutual interference or the vector cross-term between the secondary magnetic flux and the secondary driving current is cancelled out. Theoretically there are three necessary and sufficient conditions in order to decouple two vectors of the secondary magnetic flux and the secondary driving current. These conditions are usually satisfied by adding a decoupling calculation unit to the ordinary vector control system.
In the above-mentioned decoupled-vector control method or system for an induction motor, however, since the decoupling calculation unit and the 2-3 phase transformation unit are provided independently, the system configuration is rather complicated.
Further, in order to drive an induction motor in exactly the same manner as in a DC motor, four-quadrant operation (to drive the motor in the normal or reverse direction freely) in indispensable. By the way, in the vector control method, since trigonometric function (SIN and COS wave signals) are used for transforming the 2-phase synchronously-rotating coordinate primary voltages into the 2-phase fixed coordinate primary voltage, it is important to change the plus and minus signs of these trigonometric signals continuously according to the rotational direction of an induction motor in order to achieve the above-mentioned four-quadrant operation.
Further, in practice, a pulse width modulation (PWM) type inverter is often used for driving a high torque 3-phase induction motor. In the PWM type inverter, a triangular wave signal is used as carrier for generating modulation signals with respect to the reference 3-phase primary voltage. However, in the conventional vector control method and system, since the triangular wave signals must synchronize with the supply voltages and therefore the number of the triangular wave signals outputted during one period of the supply voltage decreases with decreasing frequency of the supply voltages, thus resulting in problems in that higher harmonic current increases and the response characteristics are deteriorated due to an increase in control delay or waste time caused by a long period of the triangular wave signal.
Furthermore, in the above PWM inverter control method, the frequency of the supply voltage is zero when an motor stops. Therefore, no triangular wave signals are generated and thus it is impossible to generate a PWM signal to induce the secondary magnetic flux before starting a motor. Since the secondary magnetic flux is induced with a time delay after the supply voltage has been applied to the motor, the starting response characteristics are not satisfactory.